Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Systems that read and decode bar codes employing charged coupled device (CCD) and complementary metal oxide semiconductor (CMOS) based imaging systems are typically referred to hereinafter as an imaging scanner.
Imaging scanners electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Imaging scanners that include CCD, CMOS, or other imaging configurations comprise a plurality of photosensitive elements (photosensors) or pixels typically aligned in an array pattern that could include a number of arrays. The imaging scanner can use light emitting diodes (LEDs), ambient light, or other light sources for illuminating a target object, e.g., a target bar code. An image is then reflected from the target bar code and focused through a lens of the imaging system onto the pixel array. As a result, the focusing lens generates an image from its field of view (FOV) that is projected onto the pixel array. Periodically, the pixels of the array are sequentially read out creating an analog signal representative of a captured image frame. The analog signal can be amplified by a gain factor, by for example, an operational amplifier. The amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code. The decoding circuitry can be in the form of an application specific analog circuit (ASIC) or internal circuitry or programming relating to a microprocessor.
In general an imaging scanner with a fixed focusing lens fails to provide a sharp image over a wide range of scanning distances. As a result, autofocusing lens systems are employed to refocus the image through the imaging lenses to achieve relatively sharper images over, at least a wider range of scanning distances. An automatic focusing system is described in U.S. patent application Ser. No. 10/903,792 by Carlson et al. filed Jul. 30, 2004, which is assigned to the assignee of the present invention Symbol Technologies and is incorporated herein by reference for all purposes. Typically, an autofocusing system uses an actuator that is coupled to the imaging lens for movement of the imaging lens relative to an image sensor. Alternatively, a liquid lens can be used that changes its curvature or optical power in order to allow an image to come into focus. Such solutions are complex and costly to manufacture.